The number of available anti-fungal agents is now increasing, as is the incidence of significant yeast infections. The National Committee for Clinical Laboratory Standards has recently developed a standard for yeast susceptibility testing. While there is increasing clinical interest in the results of in vitro tests to help guide clinical decision-making in selecting agents to treat yeast infections, it remains to be determined how well in vitro results correlate with clinical responsiveness. In this study, we intend to (1) compare commercially-available testing procedures such as the E-test and a colorimetric method (AccuMed International, Inc.) with results obtained by the proposed reference method, (2) determine the clinical relevance of in vitro susceptibility results by correlating in vitro and clinical results, and (3) collect clinical yeast isolates and assess any changes over time of susceptibility results. Our work to date suggests that the E-test results are difficult to interpret accurately and that the colorimetric test offers the most promise for a rapid and accurate testing procedure that is feasible to perform in a routine diagnostic laboratory. The manufacturer of the plates has resumed production of the plates, and we now think that results with these plates correlate well enough with results obtained by the reference method to use this procedure in lieu of the much more labor-intensive reference method. Given the difficulties we have had with interpreting the results obtained by the E-test, we are not planning any further work with that procedure for fungal susceptibility testing. The present rate of accrual of significant clinical isolates of yeasts has been so low that we have decided to terminate the data collection portion of this study, but we will continue to offer yeast susceptibility testing on clinical isolates as a research tool for clinical investigators.